function impedance_from_a(CONSTS, plot_data)

    use_S = CONSTS.use_S;

    phi = 0;
    m_max = 15;
    a_vec = (3:0.5:10)';
    Z_vec = zeros(size(a_vec));
    Z_odn_pl_vec = zeros(size(a_vec));
    for i = 1:size(a_vec,1)
        a = a_vec(i);

        LOCAL_CONSTS = CONSTS;
        LOCAL_CONSTS.a = a;
        LOCAL_CONSTS = update_CONSTS(LOCAL_CONSTS);
        I_full = func_I_full(phi, m_max, LOCAL_CONSTS);

        Z_vec(i) = 1 ./ I_full;

        I_odn_pl = sum_I_odn_pl(phi, LOCAL_CONSTS);
        Z_odn_pl_vec(i) = 1 ./ I_odn_pl;
    end

    if(plot_data)
        figure; plot(a_vec, abs(9*10^11*Z_vec), 'b.-', ...
                         a_vec, abs(9*10^11*Z_odn_pl_vec), 'rx-'); grid; 
        title('Abs of Z(a)'); 
        xlabel('a, cm'); ylabel('|Z(a)|');
        legend('|Z(a)|','|Z(a)| for homogen. plasma', ...
            'location', 'SouthEast');
    end

end